PatchSharp

A C# library for applying text diffs and replacements with built-in fuzzy matching. Supports OpenAI's V4A patch format and Anthropic-style str_replace operations.

Installation

dotnet add package PatchSharp

Targets netstandard2.1, net8.0, and net10.0.

Usage

All methods are on the static class PatchSharp.ApplyPatch.

Apply a V4A diff

using PatchSharp;

var original = "line one\nline two\nline three\n";
var diff = @"*** original.txt
 line one
-line two
+line 2
 line three
";

var result = ApplyPatch.Apply(original, diff);
// "line one\nline 2\nline three\n"

Create a new file from a diff

var diff = @"*** new-file.txt
+hello
+world
";

var result = ApplyPatch.Create(diff);
// "hello\nworld\n"

Find and replace with fuzzy matching

var input = "function hello() {\n  return 'world';\n}\n";

var result = ApplyPatch.StrReplace(
    input,
    oldStr: "  return 'world';",
    newStr: "  return 'hello world';");

Replace all occurrences:

var result = ApplyPatch.StrReplace(input, "old", "new", allowMulti: true);

Use a regex pattern:

var result = ApplyPatch.StrReplace(input, @"v\d+\.\d+", "v2.0", useRegex: true);

Error handling

All methods throw PatchApplyException on failure:

try
{
    ApplyPatch.Apply(input, diff);
}
catch (PatchApplyException ex)
{
    Console.WriteLine(ex.Message);
    Console.WriteLine(ex.LineNumber); // line in the diff where the error occurred
    Console.WriteLine(ex.Fuzz);      // fuzzy match level used (0 = exact)
    Console.WriteLine(ex.Context);   // surrounding context for diagnostics
}

Fuzzy matching

When an exact match isn't found, PatchSharp tries progressively looser matching:

Fuzz level	Strategy
0	Exact match
1	Ignore trailing whitespace
100	Ignore leading and trailing whitespace
500	Skip blank lines (handles AI models omitting blank lines from diff context)
1000	Unicode normalization (smart quotes → ASCII quotes, em-dashes → hyphens, etc.)

The lowest fuzz level that produces a match wins.

V4A diff format

*** path/to/file.txt           ← file header
@@ context line                ← anchor (jump to this line)
 unchanged line                ← context (space prefix)
-removed line                  ← deletion
+added line                    ← insertion
*** End of File                ← anchor to end of file

Wrapped in optional *** Begin Patch / *** End Patch markers.

Origin

PatchSharp was extracted from Instruct UI, an AI-powered platform that generates production-ready Blazor UI code from text descriptions and screenshots. These patching and replacement algorithms are battle-tested across thousands of AI-generated code edits in Instruct UI's multi-agent workflow, where reliable diff application is critical for compiling and rendering live previews.

License

MIT